DPA-Switch
DC-DC Forward
Converter Design Guide
Application Note AN-31
Introduction
The single-ended forward converter topology is usually the
best solution for DC-DC applications in industrial controls,
Telecom central office equipment, digital feature phones, and
systems that use distributed power architectures.
The feature set of
DPA-Switch
offers the following advantages
in DC-DC single-ended forward converter designs:
• Low component count
• High efficiency (typically >91% with synchronous
rectification)
• Built in soft-start to minimize stress and overshoot
• Built in accurate line under-voltage detection
• Built-in accurate line overvoltage shutdown protection
• Built in adjustable current limit
• Built-in overload and open loop fault protection
• Built-in thermal shutdown
• Programmable duty cycle reduction to limit duty cycle
excursion at high line and transient load conditions
• Very good light-load efficiency
• Selectable 300 kHz or 400 kHz operation
• Lossless integrated cycle-by-cycle current limit
The example circuits in this design guide illustrate the use of
these and other features of
DPA-Switch.
™
Scope
This document gives guidance for the design of a single-
ended forward converter with
DPA-Switch
in applications
that require a single output voltage. It is intended for systems
engineers and circuit designers who wish to become familiar
with the capabilities and requirements of
DPA-Switch
in
DC-DC applications. This application note provides background
material that will assist users of the
DPA-Switch
DC-DC forward
converter design utility that is included in the software design
tool,
PI Expert.
Subsequent application notes will provide
comprehensive procedures for designs of greater complexity.
Designers are advised to check Power Integrationsʼ website at
www.powerint.com
for the latest application information and
design tools.
Snubber
Power
Transformer
Output
Inductor
+
C
CLAMP
Bias
Voltage
Output
Capacitor
+
V
O
+
–
DC
INPUT
R
UVLO
Drain
Clamp
DPA-Switch
D
L
CONTROL
TL431 with
Frequency
Compensation
FEEDBACK CIRCUIT
R
FC
C
FC
Input
Return
PI-2873-062204
U1
C
S
F
C
BYPASS
Figure 1. Typical Configuration of DPA-Swtich in a Single-Ended DC-DC Forward Converter with One Output.
July 2004
AN-31
Description
Input
Input Voltage
Input Voltage UV Turn ON
Input Voltage UV Turn Off
Input Voltage OV Turn On
Input Voltage OV Turn Off
Output
Output Voltage
Output Ripple and Noise
Output Current
Line Regulation
Load Regulation
Transient Response Peak
Deviation
Transient Response Recovery
Total Output Power
Continuous Output Power
Efficiency
Low-Cost Design
Enhanced (non-sync rect.)
Synchronous Rectified Design
Environmental
Input-Output Isolation Voltage
Ambient Temperature
P
OUT
η
Cost
84
87
91
1500
-40
Symbol
V
IN
Min
36
29
Typ
48
Max
75
36
72
90
5.00
50
±0.2
±0.5
3
200
30
5.2
6.00
Units
VDC
VDC
VDC
VDC
VDC
V
mV
A
%
%
% of
V
OUT
µs
50-75% Load step, 100 mA/µs
48 VDC input
To 1% of final output voltage,
50-75% load step, 48 VDC input
Comment
Typical operational range
V
OUT
V
RIPPLE
I
OUT
4.8
0
±4%
20 MHz Bandwidth
W
%
%
%
VDC
°C
Measured at P
OUT
(30 W),
25
°C,
48 VDC Input
η
Enhanced
η
SyncRect
T
AMB
85
Free convection, sea level
Table 1. Typical Specifications for a Single Output DC-DC Converter.
Figure 1 shows a typical implementation of
DPA-Switch
in
a power supply with a single regulated output. This design
guide discusses considerations for selection of components
for a practical implementation of the circuit in Figure 1. It
also addresses options and tradeoffs in cost, efficiency and
complexity that include the substitution of synchronous rectifiers
and alternative generation of the bias voltage.
designer must guarantee that the converter becomes active and
fully functional at a voltage that is lower than the minimum.
Tolerance variations of the Line Undervoltage Threshold of
DPA-Switch
with prudent design margin put the practical
minimum operating voltage closer to 30 V. Similarly, the
converter must be designed to operate at voltages higher than
the maximum specified input. The actual input voltage range
should be considered to be from about 30 V to 90 V for the
typical nominal input voltage of 48 VDC.
Output Characteristics
The output voltage can be maintained to ±4% over the range of
line, load and operational temperature range with an ordinary
feedback circuit that uses a TL431 regulator. Transient response
is controlled with proper frequency compensation. The design of
the feedback network with guidance for selection of component
values is addressed in a separate section. Ripple and noise are
strongly influenced by the size of the output inductor and the
choice of output capacitors. These topics are discussed more
thoroughly later in this document.
System Requirements
The design begins with an evaluation of the requirements.
Table 1 gives the specifications for the example converters
described here, that have been constructed and evaluated as
engineering prototypes. Variants of the basic design achieve
higher efficiencies with minor increases in complexity.
Input Voltage Range
The actual input voltage range required for operation of the
converter is greater than that indicated by the specification. The
specification requires the converter to operate and to deliver
full performance at a minimum input of 36 V. Therefore, the
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AN-31
+
+
U2
D
L
DPA-Switch
C
U2
DPA-Switch
D
L
CONTROL
CONTROL
C
S
X
F
S
X
F
DC Input Derived
(a)
PI-3468-042303
Transformer Bias (unregulated)
(b)
PI-3469-042303
+
+
U2
DPA-Switch
D
L
CONTROL
U2
D
L
CONTROL
DPA-Switch
C
S
C
S
X
F
X
F
Output Coupled Inductor Bias
(c)
PI-3470-042303
Transformer Bias (regulated)
(d)
PI-3471-042303
Figure 2. Methods for Generation of Bias Voltage.
Output Rectifiers
Output rectification may be accomplished with discrete Schottky
diodes for lowest cost or synchronous rectifiers for highest
efficiency. This document discusses synchronous rectification
in greater detail in a separate section. Ultra-fast PN junction
diodes are not suitable at
DPA-Switch
operating frequencies.
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AN-31
Efficiency
Designing a DC-DC converter with
DPA-Switch
involves
several engineering tradeoffs that weigh efficiency against cost
and complexity. The circuit configuration in Figure 1 achieves
efficiencies greater than 85% over the range of input voltage
at medium loads. In typical applications without synchronous
rectifcation, approximately 25% of the total power loss will be in
the
DPA-Switch
(see
DPA-Switch
data sheet), 40% in the output
rectifiers, and 30% in the magnetic devices. The remainder is
distributed among other devices and circuit traces.
Higher efficiencies of approximately 91% can be obtained
when Schottky rectifiers are replaced by synchronous rectifiers,
allowing lower voltage drops. The efficiency can be raised
even higher with the use of the next larger device in the
DPA-Switch
family that has lower R
DS(ON)
. Further increases in
device size may not improve the efficiency due to increased
device switching losses. Losses in the magnetic devices can
be reduced by using larger cores and by switching at 300 kHz
instead of 400 kHz. All these alternatives have compromises in
size, cost and complexity that the designer must evaluate.
Temperature
DC-DC converters usually must operate over an extended
range of temperature that goes beyond the limits for ordinary
consumer electronics. Designers should be aware that the
characteristics of passive components are likely to change
significantly with temperature. Attention to these effects to
choose suitable components can prevent unexpected and
undesirable behavior.
Designers must pay particular attention to the selection of the
output capacitors and the components in the feedback circuit
to guarantee specified performance throughout the temperature
range. The details are addressed later in the sections on Output
Capacitor Selection and Feedback Design.
because the voltage across the optocoupler is controlled. This
is countered by increased complexity. Optocoupler dissipation
can be significant and should be verified. Maximum optocoupler
phototransistor current is equal to the maximum CONTROL
pin current (I
C(SKIP)
) for the selected
DPA-Switch.
Maximum
dissipation therefore occurs at the highest bias voltage (highest
input voltage for (a) and (b)) and minimum load. Table 2
provides a comparison of complexity vs performance for all
the solutions.
a) The DC input derived bias is the simplest of the three
solutions. It uses a Zener diode between the positive DC input
and the collector of the phototransistor of the optocoupler to
reduce the maximum collector-to-emitter voltage, and more
importantly, to limit the dissipation in the optocoupler. The
penalty for simplicity is a reduction in efficiency that can
be significant at high input voltages. This alternative is best
for industrial applications where the input voltage is low
(18 V to 36 V). The input voltage in industrial applications
is usually low enough to eliminate the Zener diode because
the breakdown voltages for standard optocouplers can be as
high as 70 V. Designers must check the maximum power
dissipation in the optocoupler in either case.
b) The transformer bias (unregulated) is created from a winding
on the power transformer. The forward bias winding
should be connected to the rectifier in a polarity such that it
conducts when the
DPA-Switch
is on. Since the bias voltage
is proportional to the input voltage, efficiency is reduced at
high input voltages, but the effect is less than with the direct
connection to the input. Again, the designer needs to check
the power dissipation in the optocoupler at the maximum bias
voltage. For this bias type, worst case is minimum output
load and high input voltage. Flyback bias windings are not
recommended for
DPA-Switch
applications since they will
affect the transformer reset.
c) Output coupled inductor bias uses a winding on the output
inductor to develop the bias voltage. This technique
provides a well regulated bias voltage when the converter
operates in the continuous conduction mode. Regulation
is accomplished by phasing the winding such that the bias
voltage is proportional to the output voltage by transformer
action when the
DPA-Switch
turns off. The penalty for the
higher efficiency is the cost and complexity of a custom output
inductor. The bias voltage can be adjusted by modifying turns
ratio, bias capacitor size and minimum load on the main
output. The designer should verify a minimum bias voltage
of 8 V at minimum load and maximum input voltage.
d) The transformer bias (regulated) solution peforms the
same function as the output coupled inductor bias (c). The
bias voltage regulation is not quite as good as with the
output coupled inductor bias. However, the solution does
provide a reasonably constant bias voltage over a variety
Bias Voltage
There are four ways to generate the bias voltage required for
operation of
DPA-Switch:
(a)
(b)
(c)
(d)
DC input derived
Transformer bias (unregulated)
Output coupled inductor winding
Transformer bias (regulated)
Figure 2 illustrates the four alternatives. Each one must provide
a minimum of 8 V at the collector of the optocoupler under worst
case operating conditions (minimum input voltage and minimum
load). The lowest bias voltage under typical conditions should be
12 V. The output coupled inductor and the regulated transformer
bias techniques give the highest efficiency of the four solutions
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Bias Type
DC Input
Derived Bias
Transfer Bias
(unregulated)
Output Coupled
Inductor Bias
Transformer Bias
(regulated)
Input Voltage
Range (V)
18 to 36
36 to 72
36 to 72
36 to 72
Efficiency
⇓
⇔
⇑
⇑
Cost Complexity
⇓
⇓
⇑
⇔
Comment
Recommended
for 18 to 36 V only
Recommended for
low-cost design
Only recommended if
supply already requires
coupled output inductor
Recommended for high
efficiency designs
Table 2. Bias Voltage Solution Comparison.
of input voltage and output load conditions. This solution
works best if the independent inductor is maintained in
the continuous conduction mode. The solution can be
implemented with a low current, low cost (off-the-shelf)
inductor, but the inductance value will be high enough to
ensure continuous conduction mode over the majority of
operating conditions.
50% because
DPA-Switch
uses a voltage mode control. The
quotient is the upper limit for the turns ratio.
Core and Copper
The actual number of turns for the transformer will depend
on the dimensions of the particular core. The core material
should be low loss at the
DPA-Switch
operating frequencies.
Technical data on properties of ferrite cores are available from
several suppliers. See references [1], [2] and [3]. Skin effect
and proximity effect will set a practical limit for wire size. Foil
windings become attractive when the output current is higher
than about 6 A.
Thermal considerations often dominate selection of the core.
The selection of the core is a complex trade-off between winding
area, core cross-section and ratio of core surface area to core
volume. These parameters determine the power loss as well
as the thermal resistance of the transformer. A small core may
meet the requirements in every respect except temperature
rise, forcing the use of a larger core. The only practical
way to check temperature rise is with bench evaluation of a
prototype. Temperature must be measured at the hottest spot
in the transformer, which is usually next to the center of the
core under the windings. Wire temperatures above 110
°C
need
special considerations and UL Class F materials.
Other Practical Considerations
Minimize the number of turns within the limits of other
constraints. Resistive losses depend on the length of the wire.
Maximize the amount of copper (wire) that can be fitted within
the winding window. Leakage inductance must be kept low to
reduce losses associated with clamp components. This is best
accomplished with a split primary construction that has the
secondary between the layers of the primary winding. Also,
all transformers should have no air gap.
Transformer Design
The power transformer is critical to the success of the converter
design. Requirements for efficiency, component height and
footprint will determine the details of construction. System
engineers and circuit designers may choose to specify the
electrical parameters and mechanical limits, and delegate the
construction details to a supplier of custom transformers. Use the
PI Expert
design tool to determine the proper parameters. This
section gives guidance for specification of the transformer.
Turns Ratio
The most important parameter for the power transformer is
the primary-to-secondary turns ratio. It must be low enough
to provide the regulated output voltage at the minimum input
voltage. Determine the minimum input voltage from the system
specification and the tolerance of the line under-voltage lockout
circuit.
Whereas the minimum input voltage may be specified at
36 V, worst case tolerances of the under-voltage circuit are
likely to allow the
DPA-Switch
to operate at an input as low as
29 V. From this voltage, subtract the estimated drain-to-source
voltage of
DPA-Switch
at the maximum load. Reduce it further
by an estimate of the voltage drop from the high frequency AC
resistance of the transformer windings at full load.
Multiply the result by the maximum guaranteed duty ratio and
divide by the sum of the output voltage and the drop on the
output rectifier at full load. The duty ratio can be greater than
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